Try using the request,"Hi Bixby. What is today's date?" Some members in this thread have had to rephrase the question. You are not alone and I apologize that you are experiencing this.
How did you phrase this question? Did you ask what today was when you prompted Bixby?
I had this happen to only one. You have to be very careful how you word things. I don't know why Bixby interpreted it this way at first, but when I asked again the right date was shown correctly.
So now Bixby is telling you the date is behind a day? That's weird that it changed from being one day ahead, to now being behind. Do your phone display any weird settings for date as well?